The invention concerns the technology of broadcasting an amount of general information to a multitude of radio receivers in a way that uses effectively the radio resources that are available. Especially the invention concerns the application of such technology in a cellular radio network.
In cellular networks there is commonly known the concept of cell broadcasting. It means that the base station of a cell transmits some information that is meant to be received by all mobile stations within the reach of the base station, or at least by a significant number of them. In a cell of a typical prior art cellular radio network there is determined a so-called common control channel or a BCCH (Broadcast Control Channel) that is receivable by all mobile stations in the cell. Although BCCH is a reserved name of a channel in a particular second generation cellular radio network (the GSM; Global System for Mobile telecommunications) it is used here as the general denomination of a common control channel without limiting the scope of the discussion to any particular system. The BCCH is used to convey information about e.g. the identity of the neighboring cells and their BCCH frequencies so that a mobile station may monitor the level of power it receives on those frequencies. A significant rise in the BCCH power level of a neighboring cell indicates that the mobile station is moving towards a position where a handover or cell reselection to the neighboring base station will be performed.
At the time of filing this patent application the third generation of cellular radio networks is under specification. Already it is seen that the amount of information that should be transmitted on a BCCH or the like will eventually be larger in the third generation systems than in the present second generation systems. For example, at a certain location there may be an overlap of cells from a macrocell RAN (Radio Access Network) and a nanocell RAN, whereby the mobile station should be informed not only about the identities of the neighboring cells but also about their characteristics (highest possible rate of transmitted user data, available modulation methods etc.) so that the mobile station could determine, which one of the neighboring and/or overlapping cells would be the best possible candidate for a handover or cell reselection.
A natural solution to handle the increasing amount of information to be broadcast is to increase the capacity of the BCCH by for example giving it more bandwidth or transmission power whereby in the latter case the applied channel coding could be reduced. However, radio resources (time, frequency, power) are scarce in a cell, so giving more capacity to a common channel reduces the capacity available for dedicated channels. Additionally the amount of information to be broadcast may vary from one cell to another, because not everywhere in the network are there numerous neighboring or overlapping cells, the characteristics of which should be transmitted. If a very large portion of the available capacity is given to the BCCH, it may happen that in many cells a significant portion of this capacity is underutilized while simultaneously some connection setup requests must be turned down because all capacity available for dedicated channels is occupied. Even in areas where there actually is a lot of BCCH information to be transmitted, some BCCH capacity should be reserved for future additions and this capacity would be underutilized everywhere at least during the initial service life of the system.
From the patent U.S. Pat. No. 4,850,033 there is known a method in which a transmission channel of each base station is assigned as a simplex message transmission channel in which a given base station transmits to the mobile stations information describing the structure of the control channels. Such information includes references to message transmission channels of adjoining base stations, and references to one or more control channels assigned to the given base station. The message transmission channels are thus at a higher level in the hierarchy of references than the control channels. Access by several different kinds of subscriber sets and different services can be provided by transmitting a relatively small number of references.
From the Finnish patent application number FI 980014 there is known a method in which the otherwise unused capacity of a paging channel is used to transmit information logically belonging to other channels. Since the mobile stations must listen to the paging channel regardless of whether there are paging messages or not, the periods during which their receivers are switched on can this way be utilized better than just for listening to an empty paging channel.
It is an object of this invention to present a method and a system for enabling the transmission of a variable amount of common control data in a cell. It is a further object of this invention that in a method and a system according to the invention, future additions to the common control arrangement would be possible without initially reserving significant capacity for them.
The objects of the invention are achieved by dividing the common control information into parts that are transmitted separately, and indicating, where the different parts can be found in the time and frequency co-ordinate system of the cell.
The method according to the invention is characterised in that it comprises the steps of
at a first moment of time, indicating on the first channel the location of the piece of control information on the second channel and
at a second moment of time, later than said first moment, transmitting the piece of control information on the second channel.
The general inventive idea of dividing the control channel information into parts that are separately available may be realized using two alternative approaches or a hybrid approach containing features from both of the two alternatives. The first alternative approach is to define a group of different information unit types, designated as Protocol Data Units or PDUs in an exemplary manner, and to set up a transmission schedule which relates the PDU types to certain frames of the control channel information. The most important PDU type will appear most often, preferably in each frame containing control channel information, and the other PDU types will appear in every second frame, every third frame or generally in every Nth frame according to PDU type, where N is an integer. The schedule of appearance of different PDU, types and the type of information contained within each PDU type will be communicated to the mobile stations, so they may decide, which PDU types they need to receive.
The second alternative is to send a part of the common control information on some other channel and indicating in a common control channel frame where among the other channels the additional common control information can be found. The information units contained within a common control channel frame and indicating the location of some other common control information are called pointers. The location indicated by a pointer may be for example in some traffic channel configured to be a point-to-multipoint channel. In an advantageous embodiment of the invention the pointer indicates not only the location but also the type of information contained within some other channel so that the mobile stations may decide, whether they need to receive the additional information or not.
A hybrid approach is to define a certain common control channel PDU type as a xe2x80x9cpointer PDUxe2x80x9d containing pointers, as in the second alternative discussed above, and including this PDU type among the other PDU types defined according to the first alternative discussed above. The pointer PDU will then appear regularly in every Mth common control channel frame, where M is an integer defined statically in the system specifications or dynamically, either by the base station transmitting the control channel information or by the radio network controller controlling the operation of the base station.